As a native New Yorker, it pains me to see that none of the other definitions seems to actually grasp what not for nothing means.

"Not for nothing" is used to soften the blow of something that would normally be offensive or come on too strong.

For example, if your friend showed up wearing purple and yellow striped pants to a party and asked you how he looked, you could respond, "well, not for nothing, Jimmy, but you look like a gay, retarded zebra." Or if you want to try out for a musical but you can't sing to save your life, your friend might tell you, "hey, not for nothing, but you suck at singing and you're not gonna get that part." Things of that nature.

Not for nothing, but the other guys who defined "not for nothing" are a bunch of schmucks.

It's a phrase you use towards someone when you want to stress a point that, although not really important, still bears mentioning. It's almost like when people say or write "just saying" at the end of a comment.

A girl throws a drink in your friend's face...

Friend: "I can't believe that bitch did that to me!"
You: "Not for nothing, but you were acting like an ass."

Friend: "I can't believe that bitch did that to me!"
You: "Well, you were acting like an ass. Just saying."

Colloquial, inverted way of saying, "for good reason," to emphasize a point. "Not for nothing is Paris called the City of Lights." Used extensively in English literature, including C.S. Lewis, Samuel Butler, and Jack London.

It can also imply consequences, for good or bad. "Not for nothing has Shaq played basketball all those years." Or, in a negative way, "Not for nothing has David Duke earned a reputation for bigotry."

It does not mean "for what it's worth," a common mistake. To see why this does not work, replace the words: "For what it's worth is Paris called the City of Lights."

There is a similar Italian phrase which is not used in the same manner in Italian, "non per niente."

Not for nothing had he been exposed to the pitiless struggles for life in the day of his cubhood, when his mother and he, alone and unaided, held their own and survived in the ferocious environment of the Wild. -Jack London

It is not for nothing you are named Ransom. -C.S. Lewis

Not for nothing is the Urban Dictionary known as the repository of online slang.